Summary The WestJet Airlines Boeing 737-200 (registration C-FTWJ, serial number21767) departed Kelowna Airport, British Columbia, at 0840 Pacific standard time for Vancouver International Airport, British Columbia, with 6crew and 50passengers on board. Shortly after the aircraft passed through 3300feet above sea level, the pilots heard a loud bang, similar to that of an engine compressor stall. The rpm of the number1 engine rotors reduced rapidly, and the number1 engine exhaust gas temperature reading exceeded the highest value on the indicator. The pilots shut down the number1 engine (Pratt& Whitney JT8D-17A, serial number688489) and levelled the aircraft at about 6000feet. They declared an emergency and notified Kelowna tower of their intention to return to the airport. The aircraft landed on Runway15 at 0912 with one engine inoperative and taxied back to the terminal without further incident. Airport aircraft rescue and fire-fighting services were called out but not required. There was no injury or fire. Ce rapport est galement disponible en franais. Other Factual Information General Cockpit indications were normal at all times from startup until the loud bang was heard. After the aircraft returned to Kelowna, a visual examination of the number1 engine revealed melted hardware in the turbine section. Visual examination of the fan inlet guide vanes, the 1st stage compressor, and the 2ndstage stator revealed unremarkable damage, but 3rdstage compressor damage was significant. No indications of recent ingestion of birds or other foreign objects were found. Maintenance records indicated that, during the previous day, the number 1 engine had been surging1 during start and acceleration from idle. While the aircraft was taxiing to the runway, there were at least three separate surging events and two more while taxiing back to the gate for maintenance examination and rectification. The pilots had described the surges as minor to moderate. The aircraft was removed from service. Maintenance personnel followed the Pratt& Whitney JT8D-17 Maintenance Manual, part number481671, and replaced a pressure ratio bleed control (PRBC) valve. They did not perform a boroscopic inspection for compressor damage, nor was it required by the Pratt& Whitney troubleshooting instructions written in the maintenance manual. A subsequent idle-power engine run indicated that the surging problem had been resolved and the aircraft was returned to service. The engine then operated normally for approximately 3.5hours until the incident. WestJet purchased the engine from All Nippon Airways on 02August2001. The engine was installed on C-FTWJ and maintained in accordance with existing Transport Canada regulations and directives. Engine performance had been uneventful until the reports of surging prior to the engine failure. Engine Condition Monitoring Pratt & Whitney developed an engine condition monitoring (ECM) program to track engine health, with the aim of providing an opportunity for early fault detection. Adherence to the Pratt& Whitney ECM guidelines is not mandatory, but Pratt& Whitney advises each operator to establish its own reporting and analysis procedures and alert levels for parameter shifts. Pratt&Whitney does not specify urgency or how much time should be taken to complete the analysis of the ECMdata. WestJet adopted the Pratt & Whitney ECM program and integrated it into the operation of the 737-200 fleet by recording engine parameters during the cruise portion of the flight. Generally, readings are taken and entered on a form by the flight crews at least once a day. At the end of the day, these completed forms are faxed to the operator's head office where they are manually entered into a computer. Engine ECMdata from all WestJet JT8Dengines are then reviewed by the power plant maintenance group for long-term changes in performance characteristics. If required, appropriate maintenance actions can then be implemented. WestJet also provides duplicate data from each flight to Aerothrust Corporation, based in Miami, Florida, which provides redundant performance monitoring and analysis. ECM data from the occurrence aircraft were recorded by the flight crew approximately three hours before the engine failure, but this information had yet to be passed to the WestJet power plant maintenance group for data entry. The number 1 engine ECM records from 11 March2003, the day of the incident, were analyzed after the occurrence. These records indicated that the engine exhaust gas temperature had shifted upward by 20C, accompanied by an increase in engine core speed (N2) of about 2.5percent. Although within the expected range, these upward shifts were sufficiently high to cause them to be categorized as outliers.2 To be considered statistically significant, further data points would have been required. The data recorded during the previous 30days were typical of most JT8D engine trend reports. No adverse trends were identified. The number 1 engine was removed from the aircraft and shipped to Aerothrust Corporation, an overhaul facility in Miami, Florida, for further examination. At the time of the failure, the engine had accumulated 34014hours/40551cycles since new, and 4371hours/3916cycles since the last shop visit. Representatives from the TSB, Transport Canada, WestJet, Pratt& Whitney, and the National Transportation Safety Board attended the engine tear down in Miami. All of the low-pressure compressor (LPC) and high-pressure compressor (HPC) stages, along with a box of miscellaneous loose compressor airfoils, were shipped to the TSB Engineering Laboratory in Ottawa, Canada, for further metallurgical analyses. Engine Examination The JT8D-17A engine comprises six basic sections: the front compressor (stages1 through6), also called the LPC; the rear compressor (stages7 though13), also called the HPC; the combustor; the high-pressure turbine; the low-pressure turbine; and the exhaust. All of the LPC and HPC stages, with the exception of the 1st and 2ndstages, suffered some form of mechanical distress or failure. The TSB Engineering Laboratory examined the full rotor airfoil set from the 3rdstage, using a low-to medium-power stereo microscope. Results from this examination indicated that 13airfoils had broken in fatigue; the fatigue fracture topography varied from 40percent to 85percent of the total cross-section. In addition, 14other airfoils exhibited advanced fatigue cracking. The fatigue cracks propagated from multiple origins on both the convex and concave sides of the airfoils, indicating cyclic loading in reverse bending. The stator assembly preceding the 3rdstage rotor was inspected for indications of fatigue damage. All vanes sustained heavy damage to the outboard one-third of the trailing edge. The general damage pattern consisted of tears, rips, gouges, and chipping. No indication of fatigue failure mechanism was found in any of the LPC stator stages. Extensive mechanical damage, however, could have compromised the physical evidence and masked the fatigue features. The rotors of stages 7 through 13 (HPC) were examined visually using the low-to medium-power mobile optical microscope. The stage7 rotor exhibited one airfoil broken off near the platform. The fracture showed small fatigue cracks from multiple origins on the concave side of the airfoil. The 8thstage rotor had two airfoils missing and 18airfoils broken off close to the platform. All 18broken airfoils exhibited fatigue crack progression in reverse bending, similar in nature to that exhibited by the 3rdstage rotor. The two missing airfoils from the 8th stage were recovered from the box of miscellaneous compressor airfoils. One airfoil was discoloured, had a rough surface consistent with tumbling, and had a surface that was splattered with particles. Energy dispersive X-ray analysis of the particles adhering to the surface indicated that the particles were predominantly aluminum. By contrast, the other loose stage8 airfoil had a much smoother surface of uniform color. TSB Engineering Laboratory report LP114/04 concluded that the discoloured airfoil had dislodged from the disk and was trapped somewhere downstream of the HPCstages. Close examination of the 8thstage disk slots identified one position with very light witness marks from the airfoil tongues and from the retainer; that position was likely occupied by the discoloured airfoil. One of the conclusions in TSB Engineering Laboratory report LP114/04 was as follows: "It is likely that the failure of the low-pressure compressor was preceded by the failure of the high-pressure compressor. Manufacturer's historical data seem to suggest that." Pratt & Whitney, the engine manufacturer, and Boeing conducted their own analysis of the evidence and produced a report on 18February2005. The report concluded that "An unidentified piece of broken hardware in the stator stage just preceding or just following rotor3 provided an excitation source that resulted in liberation of rotor3 airfoils and the subsequent engine failure. Other noted distress to the compressor, particularly the 7th,8th,and 9thstage rotor blades, can be explained in this scenario." Compressor Surge Troubleshooting The off-idle surge troubleshooting flow chart contained in Pratt& Whitney JT8D Maintenance Manual, part number481671, task 72-00-00-810-006, states that a boroscopic inspection is required only if damage is observed in the first stage of the compressor or the rear stage of Fractured 3rd stage compressor airfoil the turbine. In addition, the manual does not indicate that potential compressor airfoil fatigue cracking may develop as a result of a compressor stall or surge. Consequently, maintenance personnel are likely unaware that compressor airfoil fatigue cracking can occur. Another Pratt& Whitney JT8DEngine Manual (part number481672) contains sketches (reproduced in Figure1) of fracture surface profiles for 8th stage HPC airfoils, which resemble the fracture surface noted by the TSB Engineering Laboratory analysis (Photo1). This manual is intended to be used to identify these fracture surfaces only when the engine has been removed from the aircraft and disassembled. The Pratt& Whitney manual states that these fracture surfaces indicate excessive airfoil stress and that it specifically applies to engines that have been removed following engine stalls. If the engine remains on the aircraft, a boroscopic inspection will likely reveal broken airfoils, but cracks that have not resulted in airfoil failure would be difficult to detect. As well, not all stages of the compressor are accessible. However, a boroscopic inspection of the 3rd and 8th stage compressors is possible. The PRBC valve (part number 658385, serial number6154807), had accumulated 4368 hours since installation. Before the engine failure, the PRBC valve had been removed from the engine during surge troubleshooting, disassembled, and repaired. During the repair, damage was found on an adapter plug, and on the diaphragm assembly and spring. The screen on the housing was deformed. The damage noted to the PRBC components is typical of a valve that has been in service for over 4000hours and that has been subject to an unusual and rapid change in air pressures, as occurs during a compressor surge or stall. Aged PRBC valves, especially the diaphragm assembly, are not as resilient to rapid changes in air pressure. Once the damaged PRBC valve was replaced, the ground engine runs convinced the maintenance personnel that the surging problem had been resolved. However, PRBC valves, especially when new or overhauled, have slightly different operating characteristics that can mask underlying compressor problems. The Pratt& Whitney JT8D Maintenance Manual contains a flowchart to aid in troubleshooting compressor stalls and surges. The flowchart indicates that, after replacing the PRBC valve, an engine surge test should be performed. However, this particular test is not contained in the Pratt& Whitney JT8D Maintenance Manual. Absent from the flowchart is an instruction to perform "TestE - Test to Repaired Engines." TestE should be performed after replacing the PRBC valve to determine the points at which the PRBC valve will open and, therefore, the correct operation of the PRBCvalve.3 Action Taken On 02 December 2003, as a result of this incident, the TSB issued Safety Advisory A030023-1 to Transport Canada, with copies to Pratt& Whitney USA, Boeing Commercial Aircraft, and WestJet Airlines. The letter suggested a review of current aviation maintenance practices and procedures, specifically regarding engine performance troubleshooting and compressor stall incidents, to ensure that information identifying possible compressor airfoil fatigue damage is not overlooked or dismissed. Transport Canada responded by issuing Service Difficulty Advisory2004-05. This advisory strongly advises maintainers, operators, and other responsible persons that compressor surging should be given the same attention as compressor stalls. Surges should be considered minor stalls and should not be underestimated in the damage that can occur. The Advisory also stated that compressor surges and stalls can induce latent fatigue fractures culminating in engine failures. As a result of this investigation, the Pratt& Whitney JT8D Maintenance Manual (part number481671), Engine General- Troubleshooting- 04Task 72-00-00-810-06 Figure102, Sheet2, Flowchart Block9, was reworded and clarified to run "TestE" under Engine General- Adjustments/Test- 01Task 72-00-00-760-006, "Tests for Repaired Engines." This test involves determining the points at which the PRBC valve opens and closes and is more than the "idle surge check" that WestJet performed after it replaced the PRBC valve.